1. Field of the Invention
The present invention relates to an inkjet printer and inkjet printing method, in particular, it relates to an inkjet printer and inkjet printing method for printing small droplets at a high density and high frequency.
2. Description of the Related Art
Small droplets, high density nozzles and high driving frequencies have been promoted in inkjet printers. Under such circumstances, there has recently arisen a new problem called “end-deviation.”
FIG. 1 is a schematic view showing an “end-deviation.” In FIG. 1, the reference numeral 11 denotes a printing head, and the printing head 11 vertically moves while ejecting ink droplets 13 from a plurality of ejection ports arranged on an ejection port surface 14 at a high density. The ejected ink droplets 13 impact a print medium 12 to form a dot. In a high ejecting frequency of the printing head, air with viscosity surrounding the ink droplets 13 move with a movement of the ink droplet 13 flying toward the print medium 12 at a high density. As a result, a pressure in the vicinity of the ejection port surface 14 becomes smaller than that of the periphery of the printing head 11, and air surrounding the above air flows into the decompressed area in a direction shown by the arrows. The airflow especially deflects the ink droplets 13 ejected from the ejection ports positioned at both ends of an ejection port array toward the ejection ports positioned at the center thereof, and makes the ink droplets 13 impact a position deviated from a target position on the print medium 12.
FIG. 2 is a graph showing test results that the inventors performed to check the degree of the above “end-deviation.” In this case, the distance (distance to the paper) from the ejection port surface 14 to the print medium 12 was 1.3 mm, 128 ejection ports were arranged at intervals of approximately 21.2 μm, the ejection volume from each ejection port was 2.8 pl, and the ejecting frequency from each ejection port was 25 KHz. In FIG. 2, the horizontal axis indicates each arrangement position of the aligned ejection ports. In addition, the vertical axis indicates a deviation amount of a position, where the ink droplets ejected from each ejection port actually impact, from the target position. Here, in the state shown in FIG. 1, the case of impacting from the right side of the target position is shown as “+,” and the case of impacting from the left side is shown as “−.” That is, FIG. 2 reveals that the ink droplets ejected from the ejection ports at the outermost both ends are deviated to innermost sides and printed (approximately 10 μm), the deviation amount is slowly reduced as the position of the ejection port becomes close to the center, and that the print position deviation amount of the ink droplets ejected from the center ejection port becomes smallest.
FIG. 3 is a view showing a print state in the case of actually printing a uniform image with the printing head which generates such a print state. The printing head 11 mounted on a carriage moves from left to right in FIG. 3 at a predetermined speed while ejecting ink from each ejection port 31 at a fixed ejecting frequency. An image 32 formed by a first print scanning and an image 33 formed in a second print scanning are shown in FIG. 3. The ink droplets ejected from the ejection ports at the end of the printing head are deflected toward the center of the printing head to impact the print medium, and thus an area to be naturally printed by the ink droplets ejected from the ejection ports at the end appears as a blank area 34. Such a blank area 34 is generated at each connecting part between the print scans to lower the quality of a uniform image area.
The “end-deviation” is generally easily checked as the ejection volume becomes small, the ejecting frequency is high and the arrangement density of the ejection ports is high, in particular, it becomes apparent when the ejection volume is not more than 10 pl.
FIG. 4 is a graph showing a relationship between the ejection volume and the print position deviation amount examined by the inventors. Here, the horizontal axis indicates variation of the ejection volume from approximately 5 pl to 16 pl, and the vertical axis indicates the print position deviation amount of the ink droplet ejected from the ejection port at the end with use of a printing head having the same conditions as the printing head shown in FIG. 1. FIG. 4 reveals that the print position deviation amount becomes large as the ejection volume becomes small. For this reason, it is considered that, as the ink droplet becomes small, the rate of the surface area to the weight of the ink droplet is increased and the ink droplet easily receives influence from airflow.
Regarding the “end-deviation” as described above, various countermeasures have been proposed. For example, Japanese Patent Laid-Open No. 2002-096455 discloses a method for reducing the adverse effects of the “end-deviation” by providing a mask pattern to be used in performing a multi-pass printing method with features. The method will be described hereinafter.
FIG. 5 is an explanatory schematic view of the multi-pass printing method. Here, a two-pass type multi-pass printing method is shown which completes an image in an arbitrary area by two print scans. In FIG. 5, the reference numeral 1200 denotes a printing head having ejection port arrays for four colors. The printing head 1200 ejects ink droplets while moving in a main scanning direction in FIG. 5 to print dots onto the print medium.
However, in the multi-pass printing method, printing is not performed for all printable pixels by only one print scan. For example, in the two-pass type multi-pass printing, printing is performed for approximately half of all the printable pixels via the ejection ports positioned at the lower half part of the printing head 1200 in a first print scanning. And after the first print scan, the print medium is conveyed by a length corresponding to half of a print width of the printing head 1200 in a sub-scanning direction in FIG. 5.
In the subsequent second print scan, printing is performed for the remaining pixels via the ejection ports positioned at the upper half part of the printing head 1200 in the image area where the printing has already been performed for approximately half of all the pixels by the first print scan. In addition, in the second print scanning, the lower half part of the printing head 1200 performs printing for the pixels of approximately half of the blank area adjacent to the image area. When the second print scanning ends, the print medium is further conveyed by the length corresponding to a half of the print width of the printing head 1200 in the sub-scanning direction in FIG. 5.
In the two-pass type multi-pass printing method, the image is formed in stages by alternately repeating the above print main scanning for half of all the pixels and the sub-scanning of the length corresponding to half of the print width. According to the multi-pass printing method, the image is formed in the identical image area on the print medium by a plurality of print scan via the ejection port groups different from each other in the printing head. Accordingly, even if there are variations in the ejecting direction and the ejection volume of the ejection port, and even if there are some variations in conveying amount of the print medium, it is possible to make the adverse effects due to the variations inconspicuous on the image.
Moreover, although the two-pass type multi-pass printing method for completing an image by the two print main scannings is described above with reference to FIG. 5, the number of multi-pass is not limited thereto. As the number of print scannings is increased, a formed image becomes excellent in uniformity.
When the above-described multi-pass printing method is employed, a mask pattern, in which permission or non-permission of printing is determined, is frequently used in order to determine pixels for which the printing is to be performed by each print main scanning. Various image quality items other than uniformity can be improved by providing such a mask pattern with various features.
FIG. 6 is disclosed in Japanese Patent Laid-Open No. 2002-096455, and is a view showing mask patterns which are improved to avoid the end-deviation. Here, a printing head having 768 ejection ports is employed, and mask patterns used for performing four-pass type multi-pass printing is shown. The size of the mask pattern is 768 pixels corresponding to the number of ejection ports in a vertical direction, and 256 pixels in a horizontal direction. A pixel shown by black is a print permission pixel, and a pixel shown by white is a print non-permission pixel. The print permission or print non-permission of each pixel is determined so that the four mask patterns corresponding to four ejection port groups respectively are complementary to each other.
As shown in FIG. 6, a bias is provided between the numbers of print permission pixels in accordance with positions of the ejection ports. A print permission rate of the ejection port at the end is lowered compared with that of the center so that adverse effects due to impact position deviations of the ink droplets ejected from the ejection ports at the end can be made inconspicuous.
Japanese Patent Laid-Open No. 2002-096455 discloses a constitution in which the bias is provided between the numbers of print permission pixels in accordance with positions of ejection ports. Furthermore, the same Patent Document discloses that it is effective to lower the print permission rate of the ejection port positioned at the end compared with that of the ejection port positioned at the center as shown in FIG. 6 to reduce the “end-deviation.”
On the other hand, Japanese Patent Laid-Open No. 2002-292910 discloses mask patterns further advanced from the invention disclosed in Japanese Patent Laid-Open No. 2002-096455. Regarding a color inkjet printer for printing while bidirectionally moving a plurality of ejection port arrays, it is known that color unevenness arises owing to a difference between the scanning forward direction and scanning backward direction in the ink dropping order onto paper. Japanese Patent Laid-Open No. 2002-292910 aims at reducing such color unevenness and discloses mask patterns in which peaks of the print permission rates of colors are made different from each other.
On the other hand, in order to reduce the above color unevenness, a printing head has been recently provided in which the ejection port arrays of each color are arranged so as to be symmetrical in the scanning direction of the printing head. The printing head is referred to as “bidirectional head” hereinafter. The color unevenness will be briefly described hereinafter.
In the case of a general printing head, which is not the bidirectional head, ejection port arrays, in which one array is provided for every color, are generally arranged asymmetrically, and the ink dropping order to the print medium of the forward print scanning is reverse to that of the backward print scanning. For example, when a green image is printed, a print scanning for dropping yellow ink after dropping cyan ink and a print scanning for dropping cyan ink after dropping yellow ink are alternately repeated, and two kinds of green bands are alternately arranged in the sub-scanning direction. In the inkjet printing, the difference between the ink dropping order appears in a hue difference to some extent. When the hue difference can be visually recognized, the color unevenness causes an adverse effect to degrade the image. In order to avoid the adverse effects of color unevenness, the bidirectional head has been proposed in Japanese Patent Laid-Open No. 2001-171119.
FIG. 7 is a schematic view showing an example of arrangement states of the ejection port arrays in the bidirectional head. A printing head 800 has six ejection port arrays 801 to 806 each in which 128 ejection ports for ejecting ink droplets of 2.8 pl are arranged at pitches of 600 dpi. The ejection port arrays 801 and 806 eject cyan ink, the ejection port arrays 802 and 805 eject magenta ink, and the ejection port arrays 803 and 804 eject yellow ink. The two ejection port arrays (for example, 801 and 806) for ejecting the same color ink are arranged so as to deviate from each other by a half pitch (corresponding to 1200 dpi) in the sub-scanning direction. Accordingly, the printing head 800 performs ejecting operation while being moved in the main scanning direction so that an image can be formed in the sub-scanning direction at a printing resolution of 1200 dpi.
In such an arrangement of the ejection port arrays, the ink dropping order to the print medium is cyan, magenta, yellow, yellow, magenta and cyan in the forward print scanning and backward print scanning. Accordingly, the color unevenness due to the difference between the ink dropping order is prevented.
However, as the inventors carried out a diligent examination, a phenomenon was confirmed that the degrees of the end-deviation as described in the related art are different in every ejection port array in such a symmetrical type printing head.
FIG. 8 is a graph showing test results performed by the inventors. Here, the printing head shown in FIG. 7 is used in the test, and a state of the end-deviation of the ejection port arrays of each color in printing a monotone image of each color while changing a print duty is shown. As printing conditions, the distance between the ejection port surface of the printing head and the print medium (distance to the paper) was 1.15 mm, the moving speed of the carriage was 25 inch/sec, the driving frequency of the printing head was 30 KHz, and the printing resolution was 1200 dpi.
In FIG. 8, the horizontal axis indicates the print duty, and the print duty becomes 100% when the ink is ejected to all printing pixels arranged at 1200 dpi. On the other hand, the vertical axis indicates the deviation amount of the position where the ink droplets impact, the droplets being ejected from the ejection ports positioned at both ends of the ejection port array. In addition, a curved line 901 indicates a print position deviation amount of the ejection port arrays of yellow (803 and 804), and a curved line 902 indicates a print position deviation amount of the ejection port arrays of cyan (801 and 806).
As shown in FIG. 8, both the print position deviation amounts of the ejection port arrays for the two colors are increased as the printing duty is increased. However, the degrees of the deviation amounts are different from each other. That is, referring to FIG. 7 again, the two ejection port arrays of yellow (803 and 804), which are arranged adjacently to the center of the printing head, have an end-deviation amount larger than that of the two ejection port arrays of cyan (801 and 806) which are arranged away from the center. Although not shown in FIG. 8, a locus showing a print position deviation amount of the ejection port arrays of magenta arranged between the ejection port arrays of cyan and the ejection port arrays of yellow is obtained between the curved line 901 of yellow and the curved line 902 of cyan.
The above description reveals that the degree of the end-deviation has a relationship with the distance between the two ejection port arrays. As to the reason, it is considered that force for drawing the peripheral air in ejecting varies depending on an arrangement density of the ejection ports, that is, a distance between the two ejection port arrays.
FIG. 9A and FIG. 9B are schematic views each showing a relationship between the arrangement density of the ejection ports and airflow, and each shows an example of arrangement of the ejection ports for printing a single color image of 1200 dpi in the sub-scanning direction. FIG. 9A shows two ejection port arrays separated from each other at the distance d1, and FIG. 9B shows two ejection port arrays separated from each other at the distance d2 shorter than d1. In both examples, the image can be printed at the printing density of 1200 dpi in the sub-scanning direction. However, since it is considered that the amount of airflow having a risk of causing the end-deviation depends on the arrangement density of the ejection ports, it is anticipated that the amount of air flow generated under a higher arrangement density shown in FIG. 9B is larger. That is, referring to FIG. 7 again, it is assumed that the amount of airflow generated by the ink droplets ejected from the ejection port arrays of yellow (803 and 804) having an arrangement similar to that shown in FIG. 9B is larger than that from the ejection port arrays of cyan (801 and 806) having an arrangement similar to the arrangement shown in FIG. 9A, and that the end-deviation arises more easily in the arrangement shown in FIG. 9B. This is consistent with the results shown in FIG. 8.
Although image adverse effects due to the above-described end-deviation becomes apparent in full color print for printing an image with all ink colors, it becomes more apparent in mono color print for printing an image with a single color ink. This is because, in the mono color print, a contrast in a single color image is relatively high and the end-deviation can be easily checked as white streaks. When the printing head shown in FIG. 8 is employed, there arises a problem that the degree of the end-deviation, the degree of the image adverse effects, depends on the ink color to be used even in a mono color print.
No acceptable image can be obtained even when the mask patterns (shown in FIG. 6) disclosed in Japanese Patent Laid-Open No. 2002-096455 are commonly employed for all the ejection port arrays of such a printing head. When it is assumed that, for example, the mask patterns shown in FIG. 6, in which the print permission rates are extremely fluctuated, are employed for the ejection port arrays of cyan shown in FIG. 7 having a small end-deviation amount, the original effect of the multi-pass printing is lost, density variation originally present in the ejection port array is not corrected, and density unevenness is caused.
As the inventors diligently examined, they judged that, when the mask patterns disclosed in Japanese Patent Laid-Open No. 2002-096455 are employed, it is important to adjust the distribution of the print permission rates of nozzles in the ejection port array in accordance with the degree of the actual end-deviation. That is, while aiming at reducing the end-deviation, the distribution of the print permission rates in the same ejection port array is required to be determined so that new adverse effects do not arise. Accordingly, it is considered that it is necessary to determine the distribution of the print permission rates for every individual ejection port array in the case where the degrees of end-deviation of the ejection port arrays for colors are different from each other like the bidirectional printing head disclosed in Japanese Patent Laid-Open No. 2001-017111.
Japanese Patent Laid-Open No. 2002-292910 discloses mask patterns in which the print permission rates are optimized for every ejection port array. However, the mask patterns are merely provided to avoid the color unevenness, and no bidirectional printing head as shown in FIG. 7 is supposed in the invention of the above Patent Document. By using the bidirectional printing head, the color unevenness is avoided. In order to solve a new problem due to the constitution of the bidirectional printing head, the present invention aims at optimizing the print permission rates for every ejection port array.